The present invention relates generally to thermal contraction compensation for cables, and more particularly to an apparatus and method for compensating for thermal contraction of superconducting and cryo-resistive cables.
Short length demonstrator superconducting cable systems have been assembled and tested in a number of countries worldwide. Recent demonstrator cables have been of the high temperature superconductivity (HTSC) type with lengths typically less than 100 meters and up to a maximum of 200 meters. Earlier demonstrator cables were of the low temperature superconducting type and the cryo-resistive type. To be economically acceptable, commercial HTSC installations are required to have similar reel lengths (500 meters to 1,500 meters) to conventional cables to reduce the number of site assembled joints, which increase the risk of unreliable operation and which are expensive to assemble.
Conventional electric power cables with copper or aluminum conductors are typically installed at ambient temperature, e.g. 15° C. When carrying the rated current, they are designed not to exceed the specified operating temperature which is typically 90° C., a rise of 75° C. Although this increase is comparatively moderate, the expansion forces, if constrained, generate high thermomechanical forces. For example a stranded copper conductor having a cross sectional area of 2000 millimeters2 (mm2) can generate 60 kilonewtons (kN) of force. Accordingly, accessories such as joints, termination, and support structures have to be designed to withstand these forces and protect the cable system against damage. As a result, early cable systems were beset with problems of thermomechanical failure of conductor connectors in “rigidly constrained” systems and fatigue failure of a cable's metallic sheath at preferential positions in “unconstrained” systems subjected to cyclic loading.
Superconducting and cryo-resistive cables experience high thermal contraction strain when they are cooled down to their operating temperatures. The forces that are developed are sufficient to damage the cable, joints, and terminations. The conductor in a superconducting cable, particularly of the HTSC type is comprised of a large number of small and fragile elements which have low tensile strength and are difficult to connect together in a straight joint or termination in a sufficiently robust manner to withstand long term tensile forces of high magnitude. Additionally, the magnitude of current that can be carried safely in a superconducting state is limited by mechanical strain.
Insulation surrounding the conductor also contracts during cool-down and its electrical integrity is dependent upon the absence of mechanical disturbance and damage. The presence of thermal contraction strain in the cable conductor and the insulation directly impacts the feasibility and economics of HTSC cable systems in reducing the cable reel length, increasing the number of joints, reducing the current carrying capacity, and increasing the risk of electrical failure of the insulation.
Present superconducting and cryo-resistive cables operate in the temperature range of −200° C. to −270° C. The cables are installed at near room temperature, for example 15° C., and so they are required to cool-down through a large temperature drop of 215° C. to 285° C., this being some 3-4 times greater than the temperature rise experienced by conventional cables. Thus, it is apparent that the prospective thermal contraction forces experienced by the cable, joints, and terminations will be 3-4 times greater than those in conventional cable system components. Because of these forces, the amount of damage that may occur to the conductor, insulation, outer cable layers, vacuum cryostat, and coolant pressure pipes can make superconducting and cryo-resistive cables unsuitable for medium to long length commercial applications, i.e. to more than several hundred meters.